Background

Young Mercury, after he killed his father

Mercury lived in the mountains with his father, Marcus Black, who was an assassin and implied heavy drinker. Mercury was trained by his father to be an assassin, a regimen that included regular beatings and the removal of Mercury's semblance. One night, he fought against his father and killed him.

Cinder Fall and Emerald Sustrai saw their fight from afar and were hoping to recruit his father for their cause. However, due to Marcus' death, Cinder asked Mercury if he took after his father. He joined Cinder's cause and has been traveling with them since.

According to Kerry Shawcross, Mercury has one of the darkest backstories, which has yet to be fully explored.[1]

Touring Vale

After Emerald asks for directions to a local shop, Mercury appears from around a corner and jokes that he knew she was lost. Emerald pulls out a stolen wallet and offers him the money if he shuts up, but he refuses in favor of toying with her. As the duo make their way to the shop, Mercury seems bored by the city and with Emerald herself.

They arrive at the store owned by Tukson and Emerald asks him if he has certain books. Meanwhile, Mercury continues to pick up books and slams them loudly during Emerald's questioning before asking if he has any comics. When Emerald begins to reveal why they are really there, Mercury dims the windows of the shop and kills Tukson when he tries to attack them.

When Mercury returns to Roman Torchwick'shideout with Emerald, Mercury informs Roman that they were cleaning up his mess. Cinder soon arrives and Mercury confirms that they had indeed killed Tukson before he had the chance to flee. When Roman tries to explain to Cinder why he hadn't done the job himself yet because of how busy he is stealing Dust, Mercury compares him to any punk with a gun and ski mask. He then leaves the warehouse with Cinder and Emerald.

Sometime later, Mercury arrives at Beacon with Emerald and Cinder, posing as students from Haven Academy. The three bump into Ruby Rose, where Mercury informs her that they got turned around looking for their building. Ruby then tells them the direction of their building and the trio continue on their way as Ruby welcomes them to Beacon Academy.

Training Matches

Mercury vs. Pyrrha

Mercury is seen again, later on, watching Pyrrha Nikos fight with Team CRDL in a class-based training fight. Once Pyrrha finishes with the former, Mercury challenges her to a sparring match. Glynda Goodwitch briefly protests this, but Pyrrha gives her approval and the match begins.

The fight is tense, with both parties probing for weaknesses and studying their opponent's skills. Pyrrha briefly uses her Semblance of polarity to manipulate Mercury's boots, throwing him off-balance. Midway through the brief but intense match, Mercury abruptly forfeits, stating that Pyrrha, a world-renowned fighter, is completely out of his league, much to Glynda's disapproval. After the match, he briefly converses with Emerald and enthuses about how much fun he finds learning.

He is later seen discussing Pyrrha's semblance and skills with Emerald and Cinder in their dorm rooms, conveying his findings from their match.

During the Beacon Dance, Mercury enters the ballroom with Emerald and is welcomed by Yang Xiao Long, followed by his answer that both of them wouldn't miss the dance for the world.

Mercury is seen dancing with Emerald while talking to Cinder through a headpiece at certain points throughout the dance. After Cinder arrives at the dance, having successfully completed her mission, Mercury dances with her and asks how her night was, to which she responds that it was much more exciting than expected.

Attack on Vale

Mercury appears alongside Emerald and Cinder during the Grimm attack on Vale, who silently nods to the two to attack and help in taking out the Grimm attacking humans.

Shortly after the fight, Mercury, Emerald and Cinder are overlooking the city of Vale from a rooftop. When Cinder remarks on the day being successful, Mercury adds that with them having lost so much Faunus in the caves, they won't listen to them anymore. At that moment, Adam Taurus appears and reassures them that they will listen to him.

Vytal Festival Tournament

Mercury and Emerald on the festival grounds

Mercury is first seen observing different boots at a stand in the fairgrounds. Emerald cues to him as he sniffs a boot when she notes that her team is too "introverted" and "socially awkward" to have dinner with Team RWBY. After Emerald is done conversing with them, she turns to Mercury, and he asks if she got the information. She tells him Weiss and Yang will be fighting in the second round. He is later seen eating a big bowl of popcorn while spectating the match between Team JNPR and Team BRNZ along with Emerald and Cinder.

He is later seen doing push-ups in the Beacon dorms with Emerald and Cinder nearby as the latter looks through James Ironwood's files and finds information on Penny Polendina.

Mercury vs. Yang

He is elected to represent his faction in the finals round of the tournament, facing off against Yang in the first match. While the two appear to be evenly matched in the beginning, Mercury unleashed a storm of rounds at her. Just as he walks away, brushing his clothes off in confidence of his "victory", Yang rebounds from his attack and proceeds to unleash a beatdown that costs him the match.

However, he is involved in an elaborate ruse where Yang is tricked into thinking Mercury still planned on attacking her despite his lack of Aura. This causes Yang to attack him, and "fracture" his leg, the former attacking in self-defense. This allows his group to frame Yang for seemingly assaulting a non-combative opponent as the cameras showcasing the event only show Mercury standing by Yang, the latter seemingly attacking him, unprovoked. Emerald rushes to his side as he clutches his leg and cries out in pain. However, his legs are prosthetic, unbeknownst to the others present, and he can repair them himself.

He is evacuated by Cinder and Neo Politan in an Air Bus to an unknown location where he hides away so that nobody knows his leg is fine. Cinder's plan carries on with him in hiding.

At the beginning of the next tournament match, Ruby finds him in the stadium with fully healed legs. He notes that the next matchup, Pyrrha versus Penny, will be chaotic due to Pyrrha's polarity and Penny's metal body. Mercury then assumes a fighting stance, preparing to attack an unarmed Ruby. In a short conflict, Ruby attempts to both speed past Mercury, and call for help via her Scroll, to no avail, as Mercury impedes her movement, and shoots her device from her hand. The former eventually manages to evade and escape, but only arrives to see Penny's destruction at the hands of her opponent. After Penny's death, Mercury leaves Ruby to grieve without further harming her, going back the way he came.

Battle of Beacon

Mercury is seen with Cinder and Emerald, standing on a rooftop as he is filming the scene on a Scroll with glee. As Cinder instructs him to focus on the infected Atlesian Knight-200s, he willingly obeys before being interrupted by the Wyvern's arrival, which Mercury also focuses on.

After the Fall of Beacon

After the fall of Beacon, Mercury is seen present at Salem's lair surveying its surroundings and accompanying Cinder alongside Emerald. He is present as a Seer Grimm converses with Salem and as Cinder trains with her Maiden powers.

Mission to Haven Academy

Mercury, Emerald, Cinder and Arthur Watts arrive at Raven Branwen'sbandit camp under orders from Salem to convince them to cooperate with plans. Mercury introduces the group as people Raven should be afraid of, and upon Raven's dismissal, Cinder quiets Mercury's ensuing outburst. They tell Raven to bring Vernal, since she, as the Spring Maiden to Haven Academy to unlock its Relic's chamber. After negotiations, Mercury and the group return to a ship where they take rest.

Attack on Haven

Mercury loses his grip on Yang.

Mercury then accompanies Cinder as she leads the assault of Haven and confronts Yang, demanding an apology for his leg from their tournament fight. They fight, but their battle is interrupted when Cinder impales Weiss Schnee. Mercury later teams up with Emerald in stopping Yang from following Cinder to the Vault, and the two of them easily overpower her. But their advantage is cut short when Ruby rushes in to save Yang. While fighting Ruby, Mercury kicks her Crescent Rose away, mocking her for her lack of skills without her weapon. But to his surprise, she headbutts him. When Yang heads for the Vault, Mercury tries to stop her, but only grabs her detached prosthetic arm. The battle of attrition continues until the appearance of Blake Belladonna turns the tide against Mercury and his allies.

Things take a turn for the worse as Professor Lionheart retreats, much to Mercury's chagrin. Eventually, Mercury is brought to his knees, realizing his side is losing the battle. When Yang emerges from the Vault with the Relic, Mercury fears the worst for Cinder and tells Emerald they need to retreat. He and Hazel Rainart escape after Emerald overexerts her Semblance, with Hazel carrying the unconscious Emerald as the trio runs down a mountain path, their mission a failure.

After the Battle of Haven

One month after the Battle of Haven, Mercury's image is shown on a newscast as one of the perpetrators of the attack on the academy. Returning to Salem's palace, Mercury stops Tyrian Callows from provoking and terrifying Emerald before they go inside with Hazel. Later, the loss in Haven is placed solely on Cinder, and Mercury and Emerald are surprised when they learn from Salem that their master is alive, but cannot return until she redeems herself. After Hazel reveals that Ozpin is taking the Relic of Knowledge to Atlas, all of Salem's subordinates are ordered to leave the room before she unleashes her rage.

Mercury reveals his dark past.

Some time later, Mercury is training in a hallway with Emerald sitting close by. When she asks him why he joined Cinder on the night they find him, Mercury answers that it was meant to be since they needed someone with his assassin's skills. Annoyed that Emerald is dissatisfied with his answers, Mercury finds his partner distraught with the loss of Cinder, whom she considers the only family she ever had. Mercury blatantly tells her that Cinder never cared for either of them and that she is in denial with an "identity crisis".

As the two then fight each other, Mercury reveals that his father stole his Semblance once it was unlocked, and he never got it back however. He then goes on to proclaim that he feels that he is right where he should be. But when the pair is suddenly confronted by Tyrian, Mercury is easily angered when the Faunus mocks him for only knowing pain and violence and his fear of leaving this life. Mercury attacks Tyrian, only for him to be on the floor a second later. He and Emerald then learn that Tyrian and Watts are leaving for Atlas by Salem's order. They are unable to learn why, and can only look on with disgust as the Faunus and the doctor leave.

Later, Mercury spots Salem outside from the meeting room window and is later joined by Emerald. As they watch Salem create an army of Beringels with wings in horror, Hazel then explains to the two with an old saying about anyone who wants something done right should do it personally.